Imbalance between oxidative and antioxidative systems: Toward an understanding of visible light-induced titanium dioxide nanoparticles toxicity

The broad application of titanium dioxide nanoparticles (TiO2 NPs) has resulted in the release of substantial quantities of untreated TiO2 NPs into aquatic systems, which serve as the terminal sink for nanomaterials. These TiO2 NPs may induce some unexpected toxic effects to aquatic systems. The objectives of this research were to evaluate toxic effects of low dosage TiO2 NPs on Tetrahymena pyriformis under visible-light illumination conditions and further discuss possible mechanisms. The results showed that TiO2 NPs accumulated as aggregates in cytosolic vesicles of T. pyriformis. Cell numbers and viability of TiO2 NPs-exposed groups decreased to (20.2 ± 1.8)% and (17.3 ± 1.9)% after 24 h continuous 12 000 Lx illumination. Compared with the dark control, the intracellular reactive oxygen species (ROS) levels increased (1.9-fold) by the combination of 5 mg L−1 TiO2 NPs and 12 000 Lx illumination. Even incubation in the dark for 6 h after 12 h illumination, superoxide dismutase (SOD) activity of TiO2 NPs exposed groups still maintained at the high level (about 3.9-fold of dark control). Meanwhile, illumination-induced TiO2 NPs led to a reduction of scavenging hydroxyl radicals (OH) activity with an average of (14.3 ± 6.5)%. Under illumination conditions, overproduction of reactive oxygen species (ROS) by TiO2 NPs exceeded the scavenging capability of T. pyriformis and then led to the breakdown of oxidative/anti-oxidative systems, further resulting in cell growth delay and mitochondria dysfunction. All the results revealed that via visible light excitation, oxidative injury may be responsible for the toxic effects of TiO2 NPs on T. pyriformis.